Storage apparatuses comprising multiple physical storage devices are well known. Physical storage devices are hard disk drives, flash memory devices, and others. A storage apparatus generally comprises RAID groups complying with the technology of RAID (Redundant Array of Independent (or Inexpensive) Disks). A RAID group is configured of multiple physical storage devices and, in the RAID group, data is stored in accordance with a specified RAID level. With reference to the storage space of the RAID group, one or multiple logical storage devices (logical volumes) are created. The logical volumes are provided to a host computer coupled to the storage apparatus. The host computer transmits access commands (e.g. write commands or read commands) specifying the provided logical volumes to the storage apparatus. By this, to the logical volumes, data access is performed.
As a storage apparatus, a storage apparatus to which the Thin Provisioning technology is applied is well known (e.g. Patent Literature 1). According to the Thin Provisioning (also called Dynamic Provisioning), instead of logical volumes whose storage capacity is fixed, virtual volumes (hereinafter referred to as VVOLs) which are virtual logical volumes are provided to the host computer. A VVOL is a set of multiple virtual areas (virtual storage areas). If a write occurs from the host computer to the VVOL, a segment is allocated to the virtual area from the pool. Therefore, according to Thin Provisioning, the storage capacity provided to the host computer can be expanded dynamically. Note that a pool is a storage area configured of multiple segments. A segment is an actual area (an actual storage area) and, as more specifically described, for example, a storage area acquired by splitting the logical volumes configuring the pool (hereinafter referred to as PVOLs) by LBAs (Logical Block Addresses) or others. At this point, an “LBA” is an address used for specifying the position on the logical volume when the host computer reads and writes the data from and to the storage apparatus.
Meanwhile, a pool in which multiple segment groups of different performances are mixed (Patent Literature 2) is well known. Hereinafter, one segment group is referred to as a “medium.” For example, a high-performance media is configured of high-performance PVOLs, and a low-performance media is configured of low-performance PVOLs. The performance of a PVOL depends on the performance of a physical storage device which is the base of the PVOL (and/or the RAID level of the RAID group). Data migration from a certain segment in a certain medium to a segment in another medium (hereinafter referred to as segment migration) can be performed.
For operating a pool, if segments unallocated to any VVOLs are insufficient, the expansion of the storage capacity of the pool is required. A method for determining the storage capacity to be added is disclosed, for example, in the Patent Literature 3. According to the Patent Literature 3, with reference to the increase rate of segments allocated to a virtual volume, the storage capacity to run short in the future is estimated and, with reference to the result of the estimation, the storage capacity to be added is determined.